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Researchers Put the Squeeze on Polymers

Widespread opto-electronic devices such as LEDs, flat screens and solar cells rely on materials that transform electricity into light or vice versa. Organic materials built from chain-like molecules known as conjugated polymers are of a particular interest for industrial applications because they are inexpensive, easy to produce and flexible in shape.

Photos of a compressed polymer sample at pressures of 1.1 GPa (left), 7.3 GPa (center), and 31 GPa (right). A pressure of 1 Giga-Pascal (GPa) corresponds to approximately 10 000 times the atmospheric pressure. Pressure-induced structural changes were determined by X-ray diffraction. Each dot inside the rectangle represents an individual sample position and the amount of structural change is colour-coded. For 1.1 GPa, the changes are uniform across the sample, reflected by similar colors of the dots. In contrast, at 31 GPa the structural changes vary with the location on the sample. (Credit: Matti Knaapila / Institute for Energy Technology in Kjeller, Norway)

For the first time, scientists conducting research at DESY’s X-ray light source PETRA III have now demonstrated that they can reliably study structural changes in these polymers caused by extreme pressure exceeding 300 000 times the atmospheric pressure. Previous studies had been limited to one quarter of that value.

Since a polymer’s chain structure is intimately linked to its opto-electronic properties, the newly accessible pressure range may lead to the discovery of unknown polymer states with novel properties that can be exploited in future electronic devices. The study was published on Friday in the journal Macromolecules.

Squeezing polymers into new structural states

The continuous development of opto-electronic devices involves searching for conjugated polymers with novel qualities by either synthesizing new polymers or modifying existing ones. However, a newly developed polymer structure does not necessarily have desired opto-electronic properties and thus, chemical synthesis can be inefficient and costly.

The authors of the current study suggest that squeezing polymers into new structural states can help make the quest for desirable opto-electronic features more efficient. “High pressure is an elegant new way of creating structural changes in conjugated polymers,” says Matti Knaapila, the study’s lead author from the Institute for Energy Technology in Kjeller, Norway. Once a structural modification with interesting opto-electronic characteristics has been found at high pressures, it could be used to direct the subsequent chemical synthesis of polymers that behave similarly under ambient conditions.

Addressing questions regarding the pressure limit

To demonstrate the feasibility of this method, the scientists had to address a series of concerns. “High-pressure experiments are routinely used to examine minerals and rocks,” says Knaapila. “However, the approach is rarely used for conjugated polymers, which are much more fragile.” In principle, researchers would like to use the largest pressure range possible because the greater the pressure, the larger the changes it creates in the polymer structure. But how much pressure can polymers withstand? And do powerful X-rays used to analyse the structural effects of pressure damage polymers?

Moreover, previous studies had suggested that high-pressure experiments on polymers become very challenging and difficult to interpret above approximately 80 000 times the atmospheric pressure. Above this threshold, the compression of the polymer becomes uneven and causes structural changes that are no longer uniform across the sample. In addition, X-ray signals used to determine the polymer structure become weaker at larger pressures and are increasingly difficult to detect.

Pushing the pressure limit for polymers

The researchers managed to overcome these challenges using PETRA III’s Extreme Conditions Beamline. “With its highly brilliant and tightly focused X-ray beam, our beamline offers one of the foremost setups in the field of high-pressure X-ray science,” says DESY researcher Hanns-Peter Liermann who is responsible for the experimental station.

The scientists placed a polymer sample inside a pressure cell known as diamond anvil cell and gradually increased the pressure to a maximum value that was four times greater than in previous studies. At the same time, they sent PETRA III’s bright, bundled X-rays through the sample in order to examine its inner structure. Owed to the X-rays’ power and an optimized diamond anvil cell that minimizes background signals, the researchers harvested the faint signals that reveal the structural changes associated with compression.

Since the X-ray beam was only a few micrometres in size and smaller than the sample itself, the researchers were also able to scan the sample step by step and pinpoint variations of structural changes across the sample that were caused by an uneven pressure distribution inside the pressure cell.

The research team further made the crucial observation that all structural changes were fully reversible when restoring atmospheric conditions, thereby eliminating concerns that extreme pressure and intense X-ray light may damage the sample.

Enabling the discovery of new polymer states

With their work, the scientists have demonstrated for the first time that they can reliably study structural changes in conjugated polymers at unprecedented high pressures, even if the pressure is no longer uniform. “Our method makes an entirely new pressure range accessible to conjugated polymer research,” says Liermann. “In this range, we may see structural changes that are linked to new opto-electronic effects.” And these may be suitable for future electronics applications.